Papermaking machines for dewatering the paper material include a wire part, a press part and a drier part. The wire part, the press part and the drier part are arranged in the order named along the direction in which the wet paper web is transferred.
Some papermaking machines are of the type which transfers the wet paper web in open draws. The open-draw papermaking machines do not support the wet paper web with belts. As a result, the wet paper web tends to be broken in a region in which it is transferred from one section to another. Accordingly, the open-draw papermaking machines are difficult to operate at higher speeds.
In recent years, papermaking machines which are of the type for transferring the wet paper web in closed draws are prevalent in the art. The closed-draw papermaking machines have a belt for transferring the wet paper web. The wet paper web is placed on the belt and is transferred by the belt from one section to another. As a result, the closed-draw papermaking machines can operate at higher speeds and more stably.
In the closed-draw papermaking machines, the wet paper web is transferred by being transferred successively through the wire part, the press part and the drier part. In the press part, the wet paper web is transferred by the transfer belt, and is pressed by a press device to squeeze water out. Thereafter, the wet paper web is dried in the drier part.
The present applicant has proposed, in Japanese published patent application No. 2004-277971, a wet paper web transfer belt which has a first function to cause the wet paper web to stick thereon and to transfer the wet paper web and a second function to allow the wet paper web to be smoothly released from the belt for transferring the wet paper web to a next process. The wet paper web transfer belt includes a wet paper web-side layer comprising a high-polymer elastic region and a fibrous body. The fibrous body is hydrophilic and is partly exposed on the surface of the wet paper web-side layer.
As the hydrophilic fibrous body which is exposed on the surface of the wet paper web-side layer retains the water from the wet paper web, the belt performs the first function to cause the wet paper web to stick on the belt and to transfer the wet paper web. The portion of the fibrous body which is exposed on the surface of the wet paper web-side layer, so that the belt performs the second function to allow the wet paper web to be smoothly released from the belt for transferring the wet paper web to the next process.
Normally, the wet paper web transfer belt has a width which is substantially identical to the width of the press region and guide rollers. When the wet paper web transfer belt moves around the press region and the guide rollers of the papermaking machine, the wet paper web transfer belt tends to have its opposite edges, i.e., left and right edges spaced transversely across the traveling direction of the belt, and nearby portions curled between the press region and the guide rollers and also between the guide rollers themselves.
The curling of the wet paper web transfer belt is also referred to a “bimetal phenomenon” similar to the bimetal effect. The opposite edges and nearby portions of the belt are downwardly or upwardly curled. When the wet paper web transfer belt is curled, it is difficult for the wet paper web transfer belt to travel at a high speed in the papermaking machine.
The present applicant has proposed a papermaking belt which minimizes the curling of its opposite edges in Japanese published patent application No. 2000-110090.
Patent document 1: Japanese published patent application No. 2004-277971
Patent document 2: Japanese published patent application No. 2000-110090
The wet paper web transfer belt disclosed in Japanese published patent application No. 2004-277971 has both of the above two functions balanced. However, the disclosed wet paper web transfer belt does not include anything to reduce the curling of the opposite edges and nearby portions of the belt while the wet paper web transfer belt is traveling.
When part of the water contained in the wet paper web is absorbed by the hydrophilic fibrous body (e.g., rayon fibers) of the wet paper web-side layer, the fibrous body expands and then makes the wet paper web transfer belt dimensionally unstable. In recent years, particularly, since the wet paper web transfer belt is required to travel at increased speeds, it is necessary to reduce an increase in the widthwise dimension of the belt which is caused by the absorption of water by the hydrophilic fibrous body.
The papermaking belt disclosed in Japanese published patent application No. 2000-110090 has a tendency to curl in the direction of a resin layer thereof. The disclosed papermaking belt is arranged to make its opposite ends resistant to curling by having the opposite edges of the resin layer thinner than the central area of the resin layer.
However, Japanese published patent application No. 2000-110090 fails to disclose any technical solutions for achieving the above two functions for the papermaking belt, and also for bringing the center of gravity of a base fabric closely toward the wet paper web.
The present invention has been made to solve the above problems. It is an object of the present invention to provide a wet paper web transfer belt for improving a first function to cause a wet paper web to stick on the belt and to transfer the wet paper web, and a second function to allow the wet paper web to be smoothly released from the belt for transferring the wet paper web to a next process, and for reducing curling of the opposite edges and nearby portions of the wet paper web transfer belt while the wet paper web transfer belt is traveling.